It is known from one of the basic patents discussing forming of plastics (U.S. Pat. No. 4,473,665 to Martine-Vvednesky et al, issued Sep. 25, 1984) that a solution of supercritical foaming or blowing agent with a plastic melt is desired to produce foamed plastic. In the '665 patent it was suggested to saturate the plastic pellets with the foaming agent in the extruder hopper (at elevated temperature). When the saturated pellets are subsequently melted at the temperatures and pressure of the screw/barrel, the foaming agent is supercritical and mixed with the melt as it is progressed down the screw. The '665 patent preferred this approach because it said direct injection of gas into the melt in the barrel of an extruder is a very difficult task because the gas had to be metered in small quantities and it was difficult to measure the gas which could escape the barrel. Introducing the foaming agent into the hopper is, however, not practical although possible. In spite of the teachings of the '665 patent, it is known to produce foamed plastic by introducing supercritical gas into the barrel of the extruder (see e.g. U.S. Pat. No. 5,158,986 to Cha et al, issued Oct. 27, 1992). However, that approach in turn has necessitated, as predicted by the '665 patent, the development of various orifice blowing agent metering arrangements such as disclosed in international publication number WO 99/32544, published Jul. 1, 1999, which attempt to include the screw as a wiping element to prevent clogging of the very fine orifices used to meter the supercritical gas. Special valving to prevent backward flow of the foaming agent has to be also provided such as shown in international publication number WO 00/58702. While devices constructed in accordance with the teachings of the references can function, it should be apparent that the devices have to include mechanisms designed to overcome fundamental problems necessitated by introducing foaming agent as a supercritical gas into the melt. In addition, expensive gas forming equipment is required to produce the supercritical gas. Further using the screw to both melt the plastic and mix the supercritical gas with the melt is not especially effective. The mixing rate has to occur at the same rate as that at which the melt is plasticated and while a screw helix which reduced in size is especially conducive to forcing a solid into a melt, that arrangement is not necessarily an optimum mixing arrangement for mixing a solution for the melt and the supercritical blowing agent because of very high shear forces.
It has long been known in the injection molding art to use a shot-pot or accumulator or a piston/cylinder to receive melt from a continuously rotating extruder and the shot-pot is then used to inject the shot into the mold. In fact, this arrangement was replaced by the reciprocating in-line plasticating screw used in today's injection molding machine. It is know e.g. from WO publications WO 98/31521, published Jul. 23, 1998 and WO 00/26005, published May 11, 2000, to use this arrangement to produce plastic parts where the outside of the part is made from non-foamed plastic and the inside of the part made from foamed plastic or vice-versa. In the arrangements illustrated, the shot-pot (or perhaps for one illustration the screw) injects both saturated and unsaturated plastic with a mixed blowing agent and unsaturated plastics. While it is possible for composite foamed/unfoamed plastic to be produced in this manner, it is desirable for control purposes to separately control the injection of each component.